The present invention relates generally to sunlight filters, and more particularly to an aircrew controllable sunlight filter for an aircraft cockpit transparency.
The ability of aircrew members to utilize emerging technologies for aircraft instrumentation displays has been limited by the sensitivity of those displays to sunlight washout.
Unfortunately, seemingly simple solutions such as incorporating a layer of photosensitive material as part of an aircraft cockpit transparency will not work. Photosensitive materials are commonly used in sunglasses which automatically darken in response to sunlight. Photosensitive materials darken in response to the ultra-violet (UV) radiation present in sunlight. The problem is that aircraft transparencies, such as windshields and canopies, are usually made of laminated material, comprising many layers, or plies, of different materials cemented together. The different materials, and their position inside a laminate, are chosen according to their various physical and optical properties. The materials used for some of the layers are, unfortunately, structurally degraded by UV light. Therefore, the outermost layers of aircraft transparencies are deliberately chosen to filter out UV light. Because of those outermost layers, insufficient UV light will reach an inner photosensitive layer to trigger a photosensitive effect.
Another problem with typical photosensitive materials is their limited service life, primarily the number of light to dark cycles they can undergo before they lose their effectiveness. Sunglasses, unlike aircraft transparencies, generally are not left continuously outdoors and can, in any event, be inexpensively replaced.
Other solutions, such as incorporating a ply of electro-optical shutter material in combination with a photocell, mounted either inside or outside the cockpit, and a power source, are not technologically mature enough to incorporate as part of a structurally robust and affordable cockpit transparency.
Thus it is seen that there is a need for a successful method for darkening an aircraft cockpit transparency in response to sunlight to prevent or reduce sunlight washout of aircraft instrumentation displays.
It is, therefore, a principal object of the present invention to provide a practical aircrew controllable sunlight filter for an aircraft cockpit transparency.
It is a feature of the present invention that it permits localized darkening of selected areas of an aircraft cockpit transparency.
It is another feature of the present invention that it can also be used to reduce the cooling load on aircraft environmental control systems.
It is yet another feature of the present invention that it can be easily retrofitted to existing aircraft cockpit transparencies.
It is an advantage of the present invention that it selectively energizes the photosensitive filtering material so that its service life is increased.
It is another advantage of the present invention that its placement toward the inside of an aircraft cockpit transparency protects it from environmental sunlight and thus further increases its service life.
It is yet another advantage of the present invention that it is simple to understand and will be simple and straightforward to implement and to use.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.